u/Ill-Drawing662

Hey, so I'm looking for a specific book, but I don't actually know it:

A friend of mine told me she read a book once, where the pages gradually get darker, the longer the story goes. Like, the MC apparently goes mad or something, and the more insane the MC gets, the darker the pages are?

She said it starts with white pages and ends with black ones, but can't remember anything else about it, except for that she liked it and was never able to find it again. And I was wondering if maybe anyone here had any idea what the book could be?

Thanks in advance!